Heretofore, in an OFDM (orthogonal frequency division multiplexing) method which is used for digital terrestrial broadcasting, wireless LANs and the like, the influence of the MPI (multi-path interference) is reduced by inserting a guard interval which is longer than or equal to the multi-path maximum delay time into each OFDM signal (e.g., see Patent Literature 1: JP 2002-204217 A).
FIG. 1A shows a transmitting apparatus 100 which can support the conventional OFDM method; FIG. 1B shows a receiving apparatus 200 which can support the conventional OFDM method.
As shown in FIG. 1A, the transmitting apparatus 100 includes a coder 101, an interleaver 102, a symbol mapper 103, a serial-parallel converting unit (S/P) 104, an inverse fast Fourier transforming unit (IFFT) 105, a parallel-serial converting unit (P/S), 106, an insertion circuit 107 and a transmitting unit 108.
The coder 101 is configured to perform an error correction coding process on input bit streams. The interleaver 102 is configured to perform an interleaving process on the bit streams output from the coder 101. The symbol mapper 103 is configured to map the input bit streams into symbols.
The serial-parallel converting unit 104 is configured to perform a serial-parallel converting process on the symbols output from the symbol mapper 103. The inverse fast Fourier transforming unit 105 is configured to perform an inverse fast Fourier transforming process on the parallel symbols output from the serial-parallel converting unit 104. The parallel-serial converting unit 106 is configured to perform a parallel-serial converting process on the parallel symbols output from the inverse fast Fourier transforming unit 105.
The insertion circuit 107 is configured to generate OFDM signals into which guard intervals (GI) are inserted, from the serial symbols output from the parallel-serial converting unit 106. The transmitting unit 108 is configured to transmit the OFDM signals output from the insertion circuit 107.
As shown in FIG. 1B, the receiving apparatus 200 includes a receiving unit 201, a removal circuit 202, a serial-parallel converting unit (S/P) 203, a fast Fourier transforming unit (FFT) 204, a parallel-serial converting unit (P/S) 205, a symbol demapper 206, a deinterleaver 207 and a decoder 208.
The removal circuit 202 is configured to remove the guard intervals from the OFDM signals received by the receiving unit 201. The serial-parallel converting unit 203 is configured to perform a serial-parallel converting process on serial sample values of the OFDM signals output from the removal circuit 202. The fast Fourier transforming unit 204 is configured to perform a fast Fourier transforming process on the parallel sample values output from the serial-parallel converting unit 203. The parallel-serial converting unit 205 is configured to perform a parallel-serial converting process on the parallel symbols output from the fast Fourier transforming unit 204. The serial symbols output from the parallel-serial converting unit 205 are output as the bit streams input in the transmitting apparatus 100, via the symbol demapper 206, the deinterleaver 207 and the decoder 208.
FIG. 2 shows a structure of an OFDM signal transmitted in the conventional OFDM method.
As shown in FIG. 2, in the OFDM signal, a plurality of sub-carriers are arranged at 1/T intervals. In this case, since modification signals are orthogonal between the respective sub-carriers, the receiving apparatus 200 can separate and demodulate the modification signals of the respective sub-carriers by the fast Fourier transforming process.
For example, in the case where a frequency f3 has been already used by a different system (PDC system etc.), the serial-parallel converting unit 104 performs the serial-parallel converting process so that the output corresponding to the frequency f3 becomes zero.
However, as shown in FIG. 1B, the conventional receiving apparatus 200 demodulates the received OFDM signals using the fast Fourier transforming process. In the case where the symbol length in the own system is different from the symbol length in the different system, which are mixed in the same frequency bandwidth, two symbols of the different system are included within one OFDM symbol interval in the received OFDM signals.
For example, as shown in FIG. 2, OFDM symbols S33 and S34 of the different system are included in the OFDM symbol interval T of the own system which includes OFDM symbols S04, S14, S24 and S44. Here, the OFDM symbol interval T means a length of an OFDM symbol to which a guard interval has been added.
At this time, since the OFDM symbols S33 and S34 are not necessarily constructed by the same modulation signal, as shown in FIG. 2, there is a problem that the orthogonality to the OFDM symbols S04, S14, S24 and S44 is lost in the OFDM symbol interval T, and interference from the different system using the frequency f3 occurs at the output of the inverse fast Fourier transforming unit 105. Similarly, the different system suffers from the interference from systems using the OFDM method.
Moreover, in a “VSF-OFCDM (Variable Spreading Factor-Orthogonal Frequency and Code Division Multiplexing) method”, which is a so-called fourth generation communications method, symbols are separated onto a plurality of frequency axes and the symbols are spread with a spreading code having a variable spreading factor assigned to each mobile station for transmission. Therefore, a symbol length which is different from a symbol length of a different conventional system (transmission method) is used. As a result, it is expected that the orthogonality of modulation signals cannot be kept in areas where other systems are mixed, interference occurs, and the same frequency bandwidth cannot be shared.